FIG. 1 is a view illustrating an arrangement of a Multimedia Broadcast Multicast Service (hereinafter, referred to as “MBMS”) system defined by Long Term Evolution (hereinafter, referred to as “LTE”) of a mobile communication system. The following description deals with the arrangement of the MBMS system.
The MBMS system includes: an Evolved Broadcast Multicast Service Center (hereinafter, referred to as “BM-SC”) 101; an Evolved Broadcast Multicast Gateway (hereinafter, referred to as “MBMS GW”) 102; an Evolved Node Base station (hereinafter, referred to as “eNB”) 104, and a user equipment (hereinafter, referred to as “UE”) 105. The BM-SC 101 is a service control center of the MBMS system. The MBMS GW 102 employs the IP multicast method so as to carry out synchronization of timing of MBMS data transmission and transmit the MBMS data to corresponding eNBs 104. The eNB 104 manages scheduling of resources (radio resources) for single-cell MBMS transmission, and carries out MBMS data transmission. The UE 105 is a terminal device for receiving the MBMS data.
Currently, problems related to MBMS defined by LTE have been shared to a certain degree, and described in documents of 3GPP. In 3GPP TS 36.300 (E-UTRA and E-UTRAN Overall description, stage 2, v8.2.0), two basic MBMS transmission modes are defined.
One is a single-cell transmission mode (SC-PTM), in which MBMS only covers a specific cell. According to the single-cell transmission mode, the UE cannot combine radio signals transmitted from multiple cells, with each other. Accordingly, the UE in the specific cell cannot enhance the target radio signal.
Another is a multi-cell transmission mode (MBSFN), in which the UE can receive radio signals of MBMS, simultaneously transmitted from multiple cells, and can combine these radio signals with each other. This can improve reliability in reception of MBMS signals. Further, 3GPP TS 36.300 also describes MBMS reception states. The UE using MBMS can be in an RRC connected state or an RRC idle state. Which one of the MBMS reception states is set is determined by a unicast service connection state of the UE. In a case where the UE using MBMS takes part in a feedback mechanism, the UE must be in the RRC connected state.
At the 60th meeting of 3GPP TSG RAN2, held in Cheju in South Korea on Nov. 5-9, 2007, “R2-075455 (summary of discussion on MBMS reception states for SC-PTM transmission)” was proposed by companies, such as Ericsson, ETRI, and LG Electronics. This proposition reached an agreement on the reception state of the UE receiving MBMS in the single-cell transmission mode.
According to the proposition, the UE employing the single-cell MBMS transmission mode can be either in the RRC connected (RRC_CONNECTED) state or in the RRC idle (RRC_IDLE) state. The base station can request the UE employing the single-cell MBMS mode to turn to the RRC_CONNECTED state regardless of whether or not the UE is receiving a unicast service (Unicast) which supports point-to-point transmission.
Further, Ericsson proposed, in the “R2-074709 (MBMS reception state for single-cell transmission)” presented at the meeting, such a technique that the single-cell MBMS transmission is carried out only on a downlink shared channel (DL_SCH), while all of the UEs receiving MBMS are turned to the RRC_CONNECTED state. The proposition is based on a premise that there are not so many UEs receiving MBMS in the single cell. In a case where the UE receiving MBMS is turned to the RRC connected state, there are advantages in system management for the UE receiving MBMS, e.g. simplification of a mobility management process of the MBMS system, and simplification of an identification process for the UE receiving MBMS. Further, the UE receiving MBMS feeds back information on a current uplink in the unicast system, and the system optimizes a parameter for the MBMS transmission with the use of information thus fed back. Thus, the quality of MBNS can be improved. Note, however, that a large number of RRC connections cause much signaling (command) overhead of the system, and reduce resource utilization. Further, there is a limitation in the number of RRC connections available in the system. For this reason, in a case where resources of the RRC connections are occupied by a great number of the UEs receiving MBMS, the UE receiving the unicast service cannot make a connection with the system. Furthermore, due to the limitation in the number of RRC connections available in the system, there is a problem that, in a case where, for example, all of the UEs receiving MBMS in the single-cell transmission mode are turned to the RRC connected state, a maximum number of the UEs acceptable in the cell may equal to the number of RRC connections available in the system. This may reduce MBMS utilization, i.e. the number of UEs receiving MBMS is unnecessarily limited.
However, in light of current development in this technical field, it seems that MBMS will be increasingly demanded on the market, and the number of UEs receiving MBMS is expected to increase rapidly. Accordingly, it seems that the aforementioned problem will become more serious.